In ink jet systems, each drop is accurately placed on the printed substrate. The accuracy is achieved by a number of different techniques like continuous spray Inkjet and drop on demand Inkjet.
The method of Inkjet printing places certain restraints on the type and composition of inks that may be used. For instance, it is very difficult to use inks that have abrasive pigments such as metallics or titanium dioxide. Also, inks must have relatively low viscosities (generally less than approximately 30 centipoises) at the point of jetting. Thirdly, the presence of binding polymer is limited as it may cause tailing. In addition, inks containing cross-linking agents could cause blocking of the jet orifices.
Such inks, that are not useable for inkjet, are still useable in other printing technologies such as offset and flexography and in painting technologies such as airbrush, which yield less accurate results than the printing technologies.
When attempting to operate an ink jet printer to print transparent process colors on a non-white surface, a major problem is getting the right printed tint, since any substrate coloration over the non-white surface will cause color distortion owing to the transparent nature of process colors.
Current Inkjet printing systems are therefore limited to printing process colors on opaque white backgrounds, or to printing opaque spot colors only, where the large color span of the process colors is not needed.
It is possible for some applications to preprint a white background by using a different printing technology, such as Flexographic printing, but this means that a separate flexographic or other printing plate would have to be prepared and the substrate run through the printing press as a different process. In addition, objects printed by Inkjet printing may be of a variety of shapes, taking advantage of the fact that Inkjet is a non-contact printing technology, capable of printing on e.g. printing fragile surfaces. Inkjet also has the ability to print variable information and would not be suitable for all types of contact print surfaces. If the shape of the area printed by inkjet is pre-set, some of the variable information advantages of the inkjet are lost. Also, if the white tint layer is printed in a different machine than the Inkjet printer, severe color registration problems may be faced, as in applications where the white tint is only placed at the same location where the images are later located, the exact location of the white image needs to be identified by the Inkjet machine. In addition, contact printing of the white under-layer can only be done if the surface is of a regular shape.
There is need for an apparatus and method for printing an opaque white undercoat by spray or inkjet onto any flat or curved surface, wherein the shape of the white undercoat matches accurately the shape of the color image to be printed thereon.